Catalyst composition

ABSTRACT

A catalyst composition comprising at least one first support, at least one first precious metal component, at least one second support, and at least one second precious metal component. The total amount of the first precious metal component comprises from 1 to 99 weight percent based on the total of the first and second precious metal components. The average particle size of the second support is greater than the average particle size of the first support. The present invention includes a method to prepare the catalyst composition and a method to use the catalyst composition as a three-way catalyst. The composition results in a coated layer from a slurry where the more supported first precious metal component is in the bottom half and more supported second precious metal component is in the top half.

This application is a continuation of co-pending application Ser. No.08/706,480 filed Sept. 4, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalyst composition useful for thetreatment of gases to reduce contaminants contained therein and methodprocess to make the catalyst composition. More specifically, the presentinvention is concerned with improved catalysts of the type generallyreferred to as "three-way conversion" or "TWC" catalysts. These TWCcatalysts are polyfunctional in that they have the capability ofsubstantially simultaneously catalyzing the oxidation of hydrocarbonsand carbon monoxide and the reduction of nitrogen oxides.

2. Discussion of Related Art

Three-way conversion catalysts (TWC) have utility in a number of fieldsincluding the treatment of exhaust from internal combustion engines,such as automobile and other gasoline-fueled engines. Emissionsstandards for unburned hydrocarbons, carbon monoxide and nitrogen oxidescontaminants have been set by various governments and must be met, forexample, by new automobiles. In order to meet such standards, catalyticconverters containing a TWC catalyst are located in the exhaust gas lineof internal combustion engines. The catalysts promote the oxidation byoxygen in the exhaust gas of the unburned hydrocarbons and carbonmonoxide and the reduction of nitrogen oxides to nitrogen.

Known TWC catalysts which exhibit good activity and long life compriseone or more platinum group metals (e.g., platinum or palladium, rhodium,ruthenium and iridium) located upon a high surface area, refractoryoxide support, e.g., a high surface area alumina coating. The support iscarried on a suitable carrier or substrate such as a monolithic carriercomprising a refractory ceramic or metal honeycomb structure, orrefractory particles such as spheres or short, extruded segments of asuitable refractory material.

U.S. Pat. No. 4,134,860 relates to the manufacture of catalyststructures. The catalyst composition can contain platinum group metals,base metals, rare earth metals and refractory, such as alumina support.The composition can be deposited on a relatively inert carrier such as ahoneycomb.

High surface area alumina support materials, also referred to as "gammaalumina" or "activated alumina", typically exhibit a BET surface area inexcess of 60 square meters per gram ("m² /g"), often up to about 200 m²/g or more. Such activated alumina is usually a mixture of the gamma anddelta phases of alumina, but may also contain substantial amounts ofeta, kappa and theta alumina phases. It is known to utilize refractorymetal oxides other than activated alumina as a support for at least someof the catalytic components in a given catalyst. For example, bulkceria, zirconia, alpha alumina and other materials are known for suchuse. Although many of these materials suffer from the disadvantage ofhaving a considerably lower BET surface area than activated alumina,that disadvantage tends to be offset by a greater durability of theresulting catalyst.

In a moving vehicle, exhaust gas temperatures can reach 1000° C. orhigher, and such elevated temperatures cause the activated alumina (orother) support material to undergo thermal degradation caused by a phasetransition with accompanying volume shrinkage, especially in thepresence of steam, whereby the catalytic metal becomes occluded in theshrunken support medium with a loss of exposed catalyst surface area anda corresponding decrease in catalytic activity. It is a known expedientin the art to stabilize alumina supports against such thermaldegradation by the use of materials such as zirconia, titania, alkalineearth metal oxides such as baria, calcia or strontia or rare earth metaloxides, such as ceria, lanthana and mixtures of two or more rare earthmetal oxides. For example, see C. D. Keith et al U.S. Pat. No.4,171,288.

Bulk cerium oxide (ceria) is disclosed to provide an excellentrefractory oxide support for platinum group metals other than rhodium,and enables the attainment of highly dispersed, small crystallites ofplatinum on the ceria particles, and that the bulk ceria may bestabilized by impregnation with a solution of an aluminum compound,followed by calcination. U.S. Pat. No. 4,714,694 of C. Z. Wan et al,discloses aluminum-stabilized bulk ceria, optionally combined with anactivated alumina, to serve as a refractory oxide support for platinumgroup metal components impregnated thereon. The use of bulk ceria as acatalyst support for platinum group metal catalysts other than rhodium,is also disclosed in U.S. Pat. No. 4,727,052 of C. Z. Wan et al and inU.S. Pat. No. 4,708,946 of Ohata et al.

U.S. Pat. No. 4,808,564 discloses a catalyst for the purification ofexhaust gases having improved durability which comprises a supportsubstrate, a catalyst carrier layer formed on the support substrate andcatalyst ingredients carried on the catalyst carrier layer. The catalystcarrier layer comprises oxides of lanthanum and cerium in which themolar fraction of lanthanum atoms to total rare earth atoms is 0.05 to0.20 and the ratio of the number of the total rare earth atoms to thenumber of aluminum atoms is 0.05 to 0.25.

U.S. Pat. No. 4,438,219 discloses an alumina supported catalyst for useon a substrate. The catalyst is stable at high temperatures. Thestabilizing material is disclosed to be one of several compoundsincluding those derived from barium, silicon, rare earth metals, alkaliand alkaline earth metals, boron, thorium, hafnium and zirconium. Of thestabilizing materials barium oxide, silicon dioxide and rare earthoxides which include lanthanum, cerium, praseodymium, neodymium, andothers are indicated to be preferred. It is disclosed that contactingthem with a calcined alumina film permits the calcined alumina film toretain a high surface area at higher temperatures.

U.S. Pat. Nos. 4,476,246, 4,591,578 and 4,591,580 disclose three-waycatalyst compositions comprising alumina, ceria, an alkali metal oxidepromoter and noble metals. U.S. Pat. Nos. 3,993,572 and 4,157,316represent attempts to improve the catalyst efficiency of Pt/Rh based TWCsystems by incorporating a variety of metal oxides, e.g., rare earthmetal oxides such as ceria and base metal oxides such as nickel oxides.U.S. Pat. No. 4,591,518 discloses a catalyst comprising an aluminasupport with components deposited thereon consisting essentially of alanthana component, ceria, an alkali metal oxide and a platinum groupmetal. U.S. Pat. No. 4,591,580 discloses an alumina supported platinumgroup metal catalyst. The support is sequentially modified to includesupport stabilization by lanthana or lanthana rich rare earth oxides,double promotion by ceria and alkali metal oxides and optionally nickeloxide. Palladium containing catalyst compositions e.g. U.S. Pat. No.4,624,940 have been found useful for high temperature applications. Thecombination of lanthanum and barium is found to provide a superiorhydrothermal stabilization of alumina which supports the catalyticcomponent, palladium.

U.S. Pat. No. 4,294,726 discloses a TWC catalyst composition containingplatinum and rhodium obtained by impregnating a gamma alumina carriermaterial with an aqueous solution of cerium, zirconium and iron salts ormixing the alumina with oxides of, respectively, cerium, zirconium andiron, and then calcining the material at 500 to 700° C. in air afterwhich the material is impregnated with an aqueous solution of a salt ofplatinum and a salt of rhodium dried and subsequently treated in ahydrogen-containing gas at a temperature of 250-650° C. The alumina maybe thermally stabilized with calcium, strontium, magnesium or bariumcompounds. The ceria-zirconia-iron oxide treatment is followed byimpregnating the treated carrier material with aqueous salts of platinumand rhodium and then calcining the impregnated material.

U.S. Pat. No. 4,780,447 discloses a catalyst which is capable ofcontrolling HC, CO and NO_(x) as well as H₂ S in emissions from thetailpipe of catalytic converter equipped automobiles. The use of theoxides of nickel and/or iron is disclosed as an H₂ S gettering compound.

U.S. Pat. No. 4,965,243 discloses a method to improve thermal stabilityof a TWC catalyst containing precious metals by incorporating a bariumcompound and a zirconium compound together with ceria and alumina. Thisis disclosed to form a catalytic moiety to enhance stability of thealumina washcoat upon exposure to high temperature.

J01210032 (and AU-615721) discloses a catalytic composition comprisingpalladium, rhodium, active alumina, a cerium compound, a strontiumcompound and a zirconium compound. These patents suggest the utility ofalkaline earth metals in combination with ceria, and zirconia to form athermally stable alumina supported palladium containing washcoat.

U.S. Pat. Nos. 4,624,940 and 5,057,483 refer to ceria-zirconiacontaining particles. It is found that ceria can be dispersedhomogeneously throughout the zirconia matrix up to 30 weight percent ofthe total weight of the ceria-zirconia composite to form a solidsolution. A co-formed (e.g. co-precipitated) ceria-zirconia particulatecomposite can enhance the ceria utility in particles containingceria-zirconia mixture. The ceria provides the zirconia stabilizationand also acts as an oxygen storage component. The '483 patent disclosesthat neodymium and/or yttrium can be added to the ceria-zirconiacomposite to modify the resultant oxide properties as desired.

U.S. Pat. No. 4,504,598 discloses a process for producing a hightemperature resistant TWC catalyst. The process includes forming anaqueous slurry of particles of gamma or other activated alumina andimpregnating the alumina with soluble salts of selected metals includingcerium, zirconium, at least one of iron and nickel and at least one ofplatinum, palladium and rhodium and, optionally, at least one ofneodymium, lanthanum, and praseodymium. The impregnated alumina iscalcined at 600° C. and then dispersed in water to prepare a slurrywhich is coated on a honeycomb carrier and dried to obtain a finishedcatalyst.

U.S. Pat. Nos. 3,787,560, 3,676,370, 3,552,913, 3,545,917, 3,524,721 and3,899,444 all disclose the use of neodymium oxide for use in reducingnitric oxide in exhaust gases of internal combustion engines. U.S. Pat.No. 3,899,444 in particular discloses that rare earth metals of thelanthanide series are useful with alumina to form an activatedstabilized catalyst support when calcined at elevated temperatures. Suchrare earth metals are disclosed to include lanthanum, cerium,praseodymium, neodymium and others.

TWC catalyst systems comprising a carrier and two or more layers ofrefractory oxide are disclosed. One of the purposes of using catalystshaving two or more layers is to isolate constituents of compositions indifferent layers to prevent interaction of the catalysts.

Recent disclosures regarding catalysts comprising two or more layers areincluded in U.S. Serial No. 08/645,985 and in European PatentApplication Nos. 95/00235 and 95/35152.

In U.S. Ser. No. 08/645,985 a catalyst architecture is provided whereinthere are two catalyst zones. The upstream zone begins to reactoxidizable components and reducible components at a lower temperaturethan the downstream zone. Each zone can comprise a catalyst having atleast one layer. This reference discloses the different functions ofeach layer and the desirability of having different constituents of eachlayer in intimate contact. European Patent Application No. 95/35152discloses a TWC catalyst which comprises an inner and outer layer.Preferably, the inner layer comprises a palladium component whilerhodium is in the second or outer layer.

Japanese Patent Publication No. 145381/1975 discloses acatalyst-supported structure for purifying exhaust gases comprising athermally insulating ceramic carrier and at least two layers of catalystcontaining alumina or zirconia, the catalysts in the catalyst containingalumina or zirconia layers being different from each other.

Japanese Patent Publication No. 105240/1982 discloses a catalyst forpurifying exhaust gases containing at least two kinds of platinum-groupmetals. The catalyst comprises at least two carrier layers of arefractory metal oxide each containing a different platinum-group metal.There is a layer of a refractory metal oxide free from theplatinum-group metal between the carrier layers and/or on the outside ofthese carrier layers.

Japanese Patent Publication No. 52530/1984 discloses a catalyst having afirst porous carrier layer composed of an inorganic support and aheat-resistant noble metal-type catalyst deposited on the surface of thesupport and a second heat-resistant non-porous granular carrier layerhaving deposited thereon a noble metal-type catalyst, said secondcarrier layer being formed on the surface of the first carrier layer andhaving resistance to the catalyst poison.

Japanese Patent Publication No. 127649/1984 discloses a catalyst forpurifying exhaust gases, comprising an inorganic carrier substrate suchas cordierite, an alumina layer formed on the surface of the substrateand having deposited thereon at least one rare earth metal such aslanthanum and cerium and at least one of platinum and palladium, and asecond layer formed on the aforesaid first alumina-based layer andhaving deposited thereon a base metal such as iron or nickel, and atleast one rare earth metal such as lanthanum, and rhodium.

Japanese Patent Publication No. 19036/1985 discloses a catalyst forpurifying exhaust gases having an enhanced ability to remove carbonmonoxide at low temperatures, said catalyst comprising a substratecomposed, for example, of cordierite and two layers of active aluminalaminated to the surface of the substrate, the lower alumina layercontaining platinum or vanadium deposited thereon, and the upper aluminalayer containing rhodium and platinum, or rhodium and palladium,deposited thereon.

Japanese Patent Publication No. 31828/1985 discloses a catalyst forpurifying exhaust gases, comprising a honeycomb carrier and a noblemetal having a catalytic action for purifying exhaust gases, the carrierbeing covered with an inside and an outside alumina layer, the insidelayer having more noble metal adsorbed thereon than the outside layer;and a process for production of this catalyst.

Japanese Patent Publication No. 232253/1985 discloses a monolithiccatalyst for purifying exhaust gases being in the shape of a pillar andcomprising a number of cells disposed from an exhaust gas inlet sidetoward an exhaust gas outlet side. An alumina layer is formed on theinner wall surface of each of the cells, and catalyst ingredients aredeposited on the alumina layer. The alumina layer consists of a firstalumina layer on the inside and a second alumina layer on the surfaceside, the first alumina layer having palladium and neodymium depositedthereon, and the second alumina layer having platinum and rhodiumdeposited thereon.

Japanese Kokai 71538/87 discloses a catalyst layer supported on acatalyst carrier and containing one catalyst component selected from thegroup consisting of platinum, palladium and rhodium. An alumina coatlayer is provided on the catalyst layer. The coat layer contains oneoxide selected from the group consisting of cerium oxide, nickel oxide,molybdenum oxide, iron oxide and at least one oxide of lanthanum andneodymium (1-10% by wt.).

U.S. Pat. Nos. 3,956,188 and 4,021,185 disclose a catalyst compositionhaving (a) a catalytically active, calcined composite of alumina, a rareearth metal oxide and a metal oxide selected from the group consistingof an oxide of chromium, tungsten, a group IVB metal and mixturesthereof and (b) a catalytically effective amount of a platinum groupmetal added thereto after calcination of said composite. The rare earthmetals include cerium, lanthanum and neodymium.

U.S. Pat. No. 4,806,519, discloses a two layer catalyst structure havingalumina, ceria and platinum on the inner layer and aluminum, zirconiumand rhodium on the outer layer.

JP-88-240947 discloses a catalyst composite which includes an aluminalayer containing ceria, ceria-doped alumina and at least one componentselected from the group of platinum, palladium and rhodium. There is asecond layer containing lanthanum-doped alumina, praseodymium-stabilizedzirconium, and lanthanum oxide and at least one component selected fromthe group of palladium and rhodium. The two layers are placed on acatalyst carrier separately to form a catalyst for exhaust gaspurification.

Japanese Patent J-63-205141-A discloses a layered automotive catalyst inwhich the bottom layer comprises platinum or platinum and rhodiumdispersed on an alumina support containing rare earth oxides, and a topcoat which comprises palladium and rhodium dispersed on a supportcomprising alumina, zirconia and rare earth oxides.

Japanese Patent J-63-077544-A discloses a layered automotive catalysthaving a first layer comprising palladium dispersed on a supportcomprising alumina, lanthana and other rare earth oxides and a secondcoat comprising rhodium dispersed on a support comprising alumina,zirconia, lanthana and rare earth oxides.

Japanese Patent J-63-007895-A discloses an exhaust gas catalystcomprising two catalytic components, one comprising platinum dispersedon a refractory inorganic oxide support and a second comprisingpalladium and rhodium dispersed on a refractory inorganic oxide support.

U.S. Pat. No. 4,587,231 discloses a method of producing a monolithicthree-way catalyst for the purification of exhaust gases. First, a mixedoxide coating is provided to a monolithic carrier by treating thecarrier with a coating slip in which an active alumina powder containingcerium oxide is dispersed together with a ceria powder and then bakingthe treated carrier.

Next platinum, rhodium and/or palladium are deposited on the oxidecoating by a thermal decomposition. Optionally, a zirconia powder may beadded to the coating slip.

U.S. Pat. No. 4,923,842 discloses a catalytic composition for treatingexhaust gases comprising a first support having dispersed thereon atleast one oxygen storage component and at least one noble metalcomponent, and having dispersed immediately thereon an overlayercomprising lanthanum oxide and optionally a second support. The layer ofcatalyst is separate from the lanthanum oxide. The noble metal caninclude platinum, palladium, rhodium, ruthenium and iridium. The oxygenstorage component can include the oxide of a metal from the groupconsisting of iron, nickel, cobalt and the rare earths. Illustrative ofthese are cerium, lanthanum, neodymium, praseodymium, etc.

U.S. Pat. No. 5,057,483, referred to above, discloses a catalystcomposition suitable for three-way conversion of internal combustionengine, e.g., automobile gasoline engine, exhaust gases and includes acatalytic material disposed in two discrete coats on a carrier. Thefirst coat includes a stabilized alumina support on which a firstplatinum catalytic component is dispersed. The first coat also includesbulk ceria, and may also include bulk iron oxide, a metal oxide (such asbulk nickel oxide) which is effective for the suppression of hydrogensulfide emissions, and one or both of baria and zirconia dispersedthroughout as a thermal stabilizer. The second coat, which may comprisea top coat overlying the first coat, contains a co-formed (e.g.,co-precipitated) rare earth oxide-zirconia support on which a firstrhodium catalytic component is dispersed, and a second activated aluminasupport having a second platinum catalytic component dispersed thereon.The second coat may also include a second rhodium catalytic component,and optionally, a third platinum catalytic component, dispersed as anactivated alumina support.

It is a continuing goal to develop a three-way catalyst system which isinexpensive and stable. At the same time the system should have theability to oxidize hydrocarbons and carbon monoxide while reducingnitrogen oxides to nitrogen.

SUMMARY OF THE INVENTION

The present invention relates to a catalyst composition, method ofpreparing the composition and the method of using the composition.

The catalyst composition of the present invention comprises at least onefirst support, at least one first precious metal component, at least onesecond support and at least one second precious metal component. Thetotal amount of the first precious metal component comprises from 1 to99, typically from 5 to 95, more typically from 20 to 80, yet moretypically from 25 to 75 weight percent based on the total of the firstand second precious metal components. The average particle size of thesecond support is greater than the average particle size of the firstsupport. The average particle size can be measured by any suitablemeans. Preferably, the particle size is measured using a BrinkmanParticle Size Analyzer. The average particle size is reported as apercent of particles below a certain measured diameter. The averageparticle size of the first support preferably is 50% and more preferably90% of the particles below 10 micrometers and more preferably below 8micrometers. The average particle size of the second support ispreferably 50% and more preferably 80% of the particles having aparticle size below 30 and more preferably 15 micrometers. The averageparticle size of the second support is at least about 1, preferably atleast about 2 and more preferably at least about 3 micrometers greaterthan the average particle size of the first support. Preferably, theaverage particle size of the second support is from 2 to 20 micrometersand more preferably 3 to 8 micrometers greater than the average particlesize of the first support.

The use of precious metal supported on supports of different particlesize results in a particle diffusion phenomena during coating of a layerof slurry of the catalyst composition. The smaller support and materiallocated on the smaller support diffuse to the bottom half of a layersupported on a substrate resulting in a greater concentration of thesmaller particles in the bottom half of the layer than the preciousmetal supported on the larger particle second support. This results in aconcentration gradient across the thickness of a coated layer whereinthere are more smaller size particles of supported material in thebottom half of the layer and more larger size particles of supportmaterial containing precious metal in the top half of a layer. Anadvantage of using the different particle size supports is thatdifferent materials on different size supports can be segregated fromeach other by being on different supports, and can further be segregatedby particle distribution due to diffusion in the layer of the catalystcomposition which is deposited from a slurry.

The present invention is particularly useful to segregate differentprecious metals from each other. For example, the catalytic activity ofa catalyst containing both palladium and rhodium in close proximity canbe reduced by their interaction. In accordance with the prior art, theseprecious metals can be separated into different layers or on differentsupport materials to avoid this effect. However, in accordance with thecomposition of the present invention, different precious metals can belocated on different supports and the different supports such as thefirst and second supports can be of different particle size or densityso that there is a certain amount of diffusional separation of theparticles within a layer deposited from a slurry. Accordingly, in apreferred embodiment, at least one of the first precious metalcomponents and at least one of the second precious metal componentscomprise at least one precious metal not present in the other preciousmetal component. Therefore, the first precious metal component cancomprise palladium and the second precious metal component can compriserhodium.

The first and second supports can be the same or different and arepreferably selected from the group from refractory oxide materials whichmore preferably include silica, alumina and titania compounds.Particularly preferred supports are activated, high surface compoundsselected from the group consisting of alumina, silica, silica-alumina,alumino-silicates, alumina-zirconia, alumina-chromia and alumina-ceria.The catalyst composition can further comprise a nickel or ironcomponent.

Other materials which can be included in the catalyst compositioninclude at least one first rare earth metal, an oxygen storagecomposition, and optionally at least one stabilizer and optionally azirconia compound. The first rare earth metal compound can be selectedfrom the group consisting of lanthanum components and neodymiumcomponents. The oxygen storage composition can be in bulk form andpreferably comprises at least one of cerium and praseodymium compounds.Useful oxygen storage compositions can comprise a refractory oxide incombination with the oxygen storage component such as a compositioncomprising ceria as an oxygen storage component and zirconia as arefractory oxide with a preferred ceria zirconia compound being aco-formed composite comprising up to 40% by weight of ceria.

The stabilizer can be any useful stabilizer for TWC catalystcompositions with preferred stabilizers including alkaline earth metalcomponents derived from a metal selected from the group consisting ofmagnesium, barium, calcium and strontium. The catalyst compositionpreferably comprises a zirconia compound and a rare earth oxide selectedfrom lantana and neodymia.

A preferred catalyst composition comprises, based on catalyst loading ona substrate, from about 0.001 to about 0.3 g/in³ of at least one firstprecious metal component, from about 0.15 to about 2.0 g/in³ of thefirst support, from about 0.001 to about 0.3 g/in³ of at least onesecond precious metal component, from about 0.15 g/in³ to about 2.0g/in³ of the second support, from about 0.025 to about 0.5 g/in³ of atleast one alkaline earth metal components, from about 0.025 to about 0.5g/in³ of the zirconium component, and from about 0.025 to about 0.5g/in³ of at least one rare earth metal component selected from the groupconsisting of ceria metal components, lanthanum metal components andneodymium metal components. The composition can additionally compriseabout 0.0 to 5 g/in³ and preferably about 0.5 g/in³ to 3 g/in³ of anickel compound. Additionally, the composition can comprise from 0.1g/in³ to about 1.0 g/in³ of a particulate composite of zirconia andceria and optionally, a rare earth component selected from lanthanum andneodimia. The particular zirconia and ceria compound comprises from 50to 90 weight percent of zirconia and 10 to 40 weight percent ceria withup to 10 weight percent of a rare earth oxide selected from the groupconsisting of lantana, neodimia, yttria and mixtures thereof.

The catalyst composition of the present invention can be in the form ofa pellet or in the form of layer supported on a substrate. The preferredsubstrate is a honeycomb catalyst carrier which can be made of metal orceramic. The composition, in the form of a layer, is supported on thesubstrate and has an upper half and a bottom half, and wherein greaterthan fifty percent by weight of the first support and first preciousmetal component supported thereon is located in the bottom half, andgreater than fifty percent by weight of the second support and secondprecious metal component supported thereon is located in the top half.The resulting layer has greater than 50 percent, preferably greater than60 percent and more preferably greater than 75 percent by weight of thefirst precious metal in the bottom half; and correspondingly greaterthan 50 percent, preferably greater than 60 percent and more preferablygreater than 75 percent by weight of the second precious metal in theupper half.

The present invention additionally includes a method of preparing thecomposition including the steps of forming a complete slurry over liquidvehicle and the catalyst composition where the catalyst compositioncomprises at least one first precious metal component supported on atleast one first support and at least one second precious metal componentsupported on at least one second support, where the total amount offirst precious metal component relative to the second is as recitedabove and the average particle size of the second support is greaterthan the average particle size of the first support is as recited above.In the preferred embodiment, the method further comprises the steps offorming at least one first slurry comprising at least one first preciousmetal component supported on at least one first support and forming asecond slurry comprising at least one second precious metal componentsupported on at least one second support and mixing the first slurry andsecond slurry to make the complete slurry. The complete slurry can bedeposited as a layer on the substrate. There can be more than one firstslurry containing components which have a greater concentration in thebottom, and there can be more than one second slurry containingcomponents which have a greater concentration in the upper. In this way,segregation or components with the upper and lower half of the layer canbe achieved.

The method can yet further comprise the steps of fixing at least onefirst precious metal component on to at least one first support and/orthe at least one second precious metal component on the at least onesecond support. The precious metal which is fixed to the support can besegregated from components which may have a negative impact on thecatalytic activity of that precious metal on other supports in thecomposition. The fixing step can be suitable fixing steps known in theart such as chemically fixing or thermally fixing. A preferred fixingstep is to thermally fix the precious metal to the support. This ispreferably conducted in air at from 50° C. to about 550° C. from 0.5 toabout 2.0 hours. The method can additionally comprise steps of addingadditional materials to either the first slurry or the second slurryincluding materials such as at least one rare earth metal component, anoxygen storage component, at least one stabilizer and/or a zirconiacomponent.

The method of the present invention can further comprise the steps ofmaking at least one precious metal component supported on at least onefirst support and at least one second precious metal component supportedon at least one second support. This can be accomplished by mixing asolution of at least one water-soluble first precious metal componentand at least one first finely divided, high surface area, refractoryoxide support which is sufficiently dry to absorb essentially all of thesolution. The first precious metal is fixed to the first support to forma first frit of supported precious metal component. The first fritparticle size can be reduced by suitable milling means. Similarly, theprocess can include the step of separately mixing a solution of at leastone water soluble second precious metal component and at least onesecond finely divided, high surface area, refractory oxide support whichis sufficiently dried to adsorb essentially all of the solution. Thesecond precious metal can be fixed as a second support to form a secondfrit of supported precious metal component and the particle size of thesecond frit can be reduced by suitable milling means. The step of addingadditional materials to the first or second slurry can be conducted byadding the materials to a slurry selected from the group comprising of afirst slurry comprising the first frit or a second slurry comprising thesecond frit.

Finally, the method can comprise a step of coating a substrate with thecomplete slurry, preferably in a manner to form a particle distributionin the supported layer wherein the smaller particles are in the bottomportion distributed in greater concentration in the bottom half of thelayer and the larger particles are distributed in a greaterconcentration in the upper half of the layer.

The present invention enables supported particles to be segregatedwithin a single layer. This enables the avoidance of deleteriousinteraction of supported components such as precious metals with eachother and with other components which are supported on differentsupports. Additionally, this permits the application of the single layerwhich achieves the advantage of a comparable catalyst architecturehaving two or more layers. Multiple layers of the same or differentcompositions within the scope of the present invention can be appliedand advantage taken of the use of the different diameter supports andsegregation and distribution of materials within each separate layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a catalyst composition of the typeuseful as a three-way conversion catalyst or a TWC. The TWC catalystcomposite of the present invention can simultaneously catalyzes theoxidation of hydrocarbons and/or carbon monoxide and the reduction ofnitrogen oxides present in a gas stream.

The catalyst composition of the present invention is directed to amethod of using the catalyst composition and a method of preparation ofthe catalyst composition. The present invention also includes a pelletor supported layer of the catalyst composition. The layer can besupported on a suitable substrate such as a monolithic catalysthoneycomb.

The catalyst composition comprises at least one first support, at leastone first precious metal component, at least one second support, and atleast one second precious metal component. In accordance with thepresent invention the average particle size of the second support isgreater than the average particle size of the first support. The firstprecious metal can be supported on the first support and the secondprecious metal can be supported on the second support. A slurry of thecatalyst composition can result in a layer wherein there is adistribution of particles between the upper half of the layer and thebottom half of the layer. Preferably, greater than 50% by weight of thefirst support and first precious metal component supported thereon islocated in the bottom half of the layer, and greater than 50% by weightof the second support and second precious metal component supportedthereon is located in the top half of the layer. The use of separatefirst supports and second supports to support the first and secondprecious metal components results in segregation of the supportedprecious metals and other components supported on the first and secondsupport. Additionally, the use of different size refractory oxidesupports results in a distribution of the support particles when aslurry of the composition is deposited as a pellet or layer. Preferablyat least one of the first precious metal components and the at least onesecond precious metal components comprises at least one precious metalnot present in the other precious metal component. In a most preferredembodiment, the at least one of the first precious metal componentcomprises a palladium component in the absence of significant amounts ofrhodium and preferably in the absence of rhodium, and the at least onesecond precious metal component comprises a rhodium component in theabsence of significant amounts of palladium and preferably in theabsence of palladium. By a significant amount of rhodium or palladium ismeant, an amount sufficient to measurably interact with the other toreduce catalytic activity. In this way, a single layer catalyst can bedeposited which contains palladium and rhodium on separate first andsecond supports respectively and further the smaller size of the firstsupport results in a layer deposited from a slurry which has a greaterpercent of palladium supported on a first support in the bottom half ofthe layer and a greater distribution of rhodium supported on a secondsupport in the top half of the deposited layer. In accordance with thisarchitecture, the palladium and rhodium components are separated and theinteraction upon aging at high temperatures which could compromisecatalytic activity is minimized.

A gas stream containing hydrocarbons, carbon monoxide and/or nitrogenoxides initially first encounters a greater amount of the supportedsecond precious metal component which is designed to effectively reducenitrogen oxides to nitrogen and oxidize hydrocarbons while causing someoxidation of carbon monoxide. The gas then passes to a greater amount ofthe supported first precious metal component designed to convertpollutants, including the oxidation of hydrocarbons and remaining carbonmonoxide.

The supported first precious metal half of the layer results ineffective oxidation of hydrocarbons over wide temperature ranges forlong periods of time. In the preferred composite the first layercomprises a catalytically effective amount of a platinum or palladiumcomponent, preferably palladium with typically 5 to 250 g/ft³ and moretypically 15 to 100 g/ft³ and preferably 25 to 75 g/ft³ of a palladiumcomponent. Platinum can be used at from 0 to 100 g/ft³, and typically atleast 0.1 g/ft³ and more typically 0.5 to 5.0 and more preferably from 5to 75 g/ft³ by weight of platinum component.

The supported second precious metal layer preferably comprises a secondrhodium component and optionally a second platinum component. The amountof rhodium component on the second support is from 0.1 to about 20,preferably from 0.5 to 15 g/ft³. The supported second precious metalpreferably contains from 50 to 100 weight percent of the rhodiumcomponent based on the total rhodium metal in the first and secondlayers.

The performance of the first platinum group precious metal componentscan be enhanced by the use of a stabilizer, preferably alkaline earthmetals, promoters preferably selected from lanthanum and neodymium, anda zirconium component. An oxygen storage component is preferably alsoincluded. The oxygen storage component can be in any form, includingbulk form, as part of a first oxygen storage composition, or impregnatedas a solution where there can be intimate contact between the oxygenstorage component and the first layer platinum group metal components.Intimate contact occurs when the oxygen storage component is introducedin the form of a solution of a soluble salt which impregnates thesupport and other particulate material and then can be converted to anoxide form upon calcining. A useful oxygen storage composition is a bulkcomposite comprising ceria and zirconia. This results in the oxygenstorage component having minimum intimate contact with the platinumgroup metal components (i.e., the rhodium and palladium components) evenwhere the platinum group metal components are supported on the bulkoxygen storage composition particles. It is preferred to include asecond zirconium component in the composition.

The first support and second support which can be the same or differentsupport components. The support preferably comprises a high surface arearefractory oxide support. The average particle size of the secondsupport is greater than the average particle size of the first support.For the purpose of the present invention, particle size is measuredusing a Brinkman particle size analyzer. The particle size distributionis indicated by a percent of particles having an average particlediameter less than a given number in micrometers. Typically, theparticles of the first support and second support have at least 80% ofthe particles having an average diameter of less than 20 microns andpreferably the first support has 90% of the particles having an averagediameter of less than 15 micrometers and the second support has at least80% of the particles having an average diameter of less than 25micrometers. Nominally, particles of precious metal and other componentssupported on a support are considered to have the same particle size asthe support.

Preferably, a first support supporting a precious metal componentcomprises a refractory oxide such as a mixture of high surface areaaluminas supporting a precious metal component comprising palladium hasa preferred particle size of 90% of the particles being less than 8 to12 microns and a second support supporting a precious metal componentcomprises a mixture of high surface area alumina and co-formed ceriazirconia has an average particle size of 80% of the particles being lessthan 10 to 14 micrometers.

Useful high surface area supports include one or more refractory oxides.These oxides include, for example, silica and alumina, include mixedoxide forms such as silica-alumina, aluminosilicates which may beamorphous or crystalline, alumina-zirconia, alumina-chromia,alumina-ceria and the like. The support is substantially comprised ofalumina which preferably includes the members of the gamma ortransitional alumina, such as gamma and eta aluminas, and, if present, aminor amount of other refractory oxide, e.g., about up to 20 weightpercent. Desirably, the active alumina has a specific surface area of 60to 350 m² /g.

The preferred catalyst of this invention comprises platinum group metalcomponents present in an amount sufficient to provide compositionshaving significantly enhanced catalytic activity to oxidize hydrocarbonsand carbon monoxide and reduce nitrogen oxides. The location of theplatinum group metal components, particularly the rhodium component andpalladium component and the relative amounts of rhodium components inthe respective first and second layers have been found to affect thedurability of catalyst activity.

In preparing the catalyst, a precious metal component such as a platinumgroup metal catalytic component can be a suitable compound, and/orcomplex of any of the platinum group metals may be utilized to achievedispersion of the catalytic component on the support, preferablyactivated alumina and/or ceria-zirconia composite support particles. Asused herein, the term "precious metal components" include gold, silverand "platinum group metal component" including the recited platinum,rhodium, platinum, ruthenium and iridium components and means any suchplatinum group metal compound, complex, or the like which, uponcalcination or use of the catalyst decomposes or otherwise converts to acatalytically active form, usually, the metal or the metal oxide. Watersoluble compounds or water dispersible compounds or complexes of one ormore platinum group metal components may be utilized as long as theliquid used to impregnate or deposit the catalytic metal compounds ontothe support particles does not adversely react with the catalytic metalor its compound or complex or the other components of the slurry, and iscapable of being removed from the catalyst by volatilization ordecomposition upon heating and/or the application of vacuum. In somecases, the completion of removal of the liquid may not take place untilthe catalyst is placed into use and subjected to the high temperaturesencountered during operation. Generally, both from the point of view ofeconomics and environmental aspects, aqueous solutions of solublecompounds or complexes of the platinum group metals are preferred. Forexample, suitable compounds are chloroplatinic acid, amine solubilizedplatinum hydroxide such as hexahydroxymonoethanolamine complexes ofplatinum, rhodium chloride, rhodium nitrate, hexamine rhodium chloride,palladium nitrate or palladium chloride, etc. During the calcinationstep, or at least during the initial phase of use of the catalyst, suchcompounds are converted into a catalytically active form of the platinumgroup metal or a compound thereof, typically an oxide.

The preferred palladium component on the first support used in a layerto coat a monolithic honeycomb substrate comprises a loading of from 5to 250 g/ft³. Regardless of the loading of the palladium, the amount ofthe high surface area alumina used will result in a palladium areaconcentration between 0.2 to 0.6 mg/m², preferably about 0.4 mg/m². Theoptimum or preferred loading of rhodium as a second precious metal on asecond support of alumina and ceria-zirconia composite is approximately2 to 20 g/ft³ and preferably 4 to 8 g/ft³ which corresponds to about0.05 to 0.1 mg/m², (depending on the distribution of alumina andceria-zirconia) where there is a combination of second support materialspreferably includes alumina having a surface area of 160 m² /g andceria-zirconia having a surface area of 60 m² /g. Preferably, 50% byweight of the rhodium is deposited on the alumina at about 0.09 g/in³.

It is known that the palladium component and the rhodium component willinteract with each other and form an alloy if in contact, losing theircatalytic activity at high temperature. The present composition can bein the form of a single coat with the palladium and the rhodiumpreferably fixed on different supports and more preferably supportshaving different particle size.

The catalyst composition of the present invention preferably contains anoxygen storage component which can be in bulk form or in intimatecontact with the supported precious metal component, i.e., palladium andrhodium. The oxygen storage component is any such material known in theart and preferably at least one oxide of a metal selected from the groupconsisting of rare earth metals, most preferably a cerium orpraseodymium compound with the most preferred oxygen storage componentbeing cerium oxide (ceria).

The oxygen storage component can be included by dispersing methods knownin the art. Such methods can include impregnation onto the first orsecond support composition. The oxygen storage component can be in theform of an aqueous solution. Drying and calcining the resulted mixturein air results in an oxide of the oxygen storage component in intimatecontact with the platinum group metal component. Typically, impregnationmeans that there is substantially sufficient liquid to fill the pores ofthe material being impregnated. Examples of water soluble, decomposableoxygen storage components which can be used include, but are not limitedto, cerium acetate, praseodymium acetate, cerium nitrate, praseodymiumnitrate, etc. U.S. Pat. No. 4,189,404 discloses the impregnation ofalumina based support composition with cerium nitrate.

Alternatively, the oxygen storage composition can be in bulk form. Thebulk oxygen storage composition can comprise an oxygen storage componentwhich is preferably a cerium group component preferably ceria orpraseodymia, and most preferably ceria. By bulk form it is meant thatthe composition comprising ceria and/or praseodymia is present asdiscrete particles which may be as small as 0.1 to 15 microns indiameter or smaller, as opposed to having been dispersed in solution asin the first layer. A description and the use of such bulk components ispresented in U.S. Pat. No. 4,714,694, hereby incorporated by reference.As noted in U.S. Pat. No. 4,727,052, also incorporated by reference,bulk form includes oxygen storage composition particles of ceria admixedwith particles of zirconia, or zirconia activated alumina. It isparticularly preferred to dilute the oxygen storage component as part ofan oxygen storage component composition.

The oxygen storage component composition can comprise an oxygen storagecomponent, preferably ceria and a diluent component. The diluentcomponent can be any suitable filler which is inert to interaction withplatinum group metal components so as not to adversely affect thecatalytic activity of such components. A useful diluent material is arefractory oxide with preferred refractory oxides being of the same typeof materials recited below for use as catalyst supports. Most preferredis a zirconium compound with zirconia most preferred.

Therefore, a preferred oxygen storage component is a ceria-zirconiacomposite. There can be from 1 to 99, preferably 1 to 50, morepreferably 5 to 30 and most preferably 10 to 25 weight percent ceriabased on the ceria and zirconia. Another preferred oxygen storagecomposition can comprise a composite comprising zirconia, ceria and atleast one rare earth oxide. Such materials are disclosed for example inU.S. Pat. Nos. 4,624,940 and 5,057,483, hereby incorporated byreference. Particularly preferred are particles comprising greater than50% of a zirconia-based compound and preferably from 60 to 90% ofzirconia, from 10 to 30 wt. % of ceria and optionally up to 10 wt. %,and when used at least 0.1 wt. %, of a non-ceria rare earth oxide usefulto stabilize the zirconia selected from the group consisting oflanthana, neodymia and yttria.

The composition optionally and preferably comprises a component whichimparts stabilization. The stabilizer can be selected from the groupconsisting of alkaline earth metal compounds. Preferred compoundsinclude compounds derived from metals selected from the group consistingof magnesium, barium, calcium and strontium. It is known from U.S. Pat.No. 4,727,052 that support materials, such as activated alumina, can bethermally stabilized to retard undesirable alumina phase transformationsfrom gamma to alpha at elevated temperatures by the use of stabilizersor a combination of stabilizers. While a variety of stabilizers aredisclosed, the composition of the present invention preferably usealkaline earth metal components. The alkaline earth metal components arepreferably alkaline earth metal oxides. In particularly preferredcompositions, it is desirable to use strontium oxide and/or barium oxideas the compound in the composition. The alkaline earth metal can beapplied in a soluble form which upon calcining becomes the oxide. It ispreferred that the soluble barium be provided as barium nitrite orbarium hydroxide and the soluble strontium provided as strontium nitrateor acetate, all of which upon calcining become the oxides.

In other aspects of the invention, one or more modifiers may be appliedto the activated alumina either before or after the alumina particlesare formed into an adherent, calcined coating on the carrier substrate.(As used herein, a "precursor", whether of a thermal stabilizer, orother modifier or other component, is a compound, complex or the likewhich, upon calcining or upon use of the catalyst, will decompose orotherwise be converted into, respectively, a thermal stabilizer, othermodifier or other component.) The presence of one or more of the metaloxide thermal stabilizers typically tends to retard the phase transitionof high surface area aluminas such as gamma and eta aluminas toalpha-alumina, which is a low surface area alumina. The retardation ofsuch phase transformations tend to prevent or reduce the occlusion ofthe catalytic metal component by the alumina with the consequentdecrease of catalytic activity.

In the composition, the amount of thermal stabilizer combined with thealumina may be from about 0.05 to 30 weight percent, preferably fromabout 0.1 to 25 weight percent, based on the total weight of thecombined alumina, stabilizer and catalytic metal component.

The composition can contain a compound derived from zirconium,preferably zirconium oxide. The zirconium compound can be provided as awater soluble compound such as zirconium acetate or as a relativelyinsoluble compound such as zirconium hydroxide. There should be anamount sufficient to enhance the stabilization and promotion of therespective compositions.

The composition preferably contains at least one first promoter selectedfrom the group consisting of lanthanum metal components and neodymiummetal components with the preferred components being lanthanum oxide(lanthana) and neodymium oxide (neodymia). In a particularly preferredcomposition, there is lanthana and optionally a minor amount ofneodymia. While these compounds are disclosed to act as stabilizers,they can also act as reaction promoters. A promoter is considered to bea material which enhances the conversion of a desired chemical toanother. In a TWC the promoter enhances the catalytic conversion ofcarbon monoxide and hydrocarbons into water and carbon dioxide andnitrogen oxides into nitrogen and oxygen.

The lanthanum and/or neodymium are in the form of their oxides.Preferably, these compounds are initially provided in a soluble formsuch as an acetate, halide, nitrate, sulfate or the like to impregnatethe solid components for conversion to oxides. It is preferred that inthe promoter be in intimate contact with the other components in thecomposition including and particularly the platinum group metal.

The composition of the present invention can contain other conventionaladditives such as sulfide suppressants, e.g., nickel or iron components.If nickel oxide is used, an amount from about 1 to 25% by weight of thefirst coat can be effective, as disclosed in commonly owned Ser. No.07/787,192, hereby incorporated by reference.

A particularly useful catalyst composition of the present inventioncomprises from about 0.001 to 0.3 g/in³ of a first precious metal suchas a palladium component; from about 0 to 0.01 g/in³ of the firstplatinum component; from about 0.15 to about 1.5 g./in³ of the firstsupport, i.e., alumina; from about 0.0 to 0.02 g/in³ of a secondplatinum component and from about 0.001 to 0.02 g/in³ of the rhodiumcomponent as a second precious metal component and from about 0.1 g/in³to about 1.5 g/in³ of the second support, i.e., alumina andceria-zirconia component; at least about 0.05 g/in³ and preferably fromabout 0.1 to about 1.0 g/in³ of an oxygen storage component, preferablya composite of ceria and zirconia; from about 0.01 to about 0.5 g/in³ ofat least one first alkaline earth metal components; from about 0.025 toabout 0.5 g/in³ of a zirconium component; and from about 0.0 to about0.5 g/in³ of at least one first rare earth metal component selected fromthe group consisting of lanthanum metal components and neodymium metalcomponents. The composition can further comprise from about 0.025 g/in³to about 0.5 g/in³ of a nickel component. The particulate composite ofzirconia and ceria can comprise 50 to 90 wt. % zirconia, 10 to 40 wt. %ceria and from 0 to 10 wt % rare earth oxides comprising lanthana,neodymia and mixtures thereof. Components other than the supports andprecious metal components can be added to the first or second slurries.

The catalyst composition can be coated as a layer on a monolithicsubstrate generally which can comprise from about 0.50 to about 6.0,preferably about 1.0 to about 5.0 g/in³ of catalytic composition basedon grams of composition per volume of the monolith.

The catalyst composition of the present invention can be made by anysuitable method. A preferred method comprises mixing a first mixture ofa solution of at least one water-soluble, first palladium component andoptionally a first platinum component, and finely-divided, high surfacearea, refractory oxide which is sufficiently dry to absorb essentiallyall of the solution to form a first slurry. The first palladium andoptionally platinum component are preferably comminuted in the firstslurry. Preferably, the slurry is acidic, having a pH of less than 7 andpreferably from 2 to 7. The pH is preferably lowered by the addition ofan acid, preferably acetic acid to the slurry. In particularly preferredembodiments the first slurry is comminuted to result in substantiallyall of the solids having particle sizes of less than about 10micrometers in average diameter. The first supported palladium componentand optional platinum component in the resulting first slurry can beconverted to a water insoluble form by a fixing step. The palladium andplatinum components can be converted to insoluble form thermally,chemically or by calcining. The first layer can be thermally fixed inair at preferably at about 50° C. to 550° C. for from 0.5 to 2.0 hours.

A second mixture of a solution of at least one water-soluble secondrhodium component and optionally at least one water-soluble platinumcomponent, and finely-divided, high surface area, refractory oxide whichis sufficiently dried to absorb essentially all of the solution ismixed. The second platinum component and second rhodium component areadded to water to form a second slurry and preferably comminuted in thesecond slurry. Preferably, the second slurry is acidic, having a pH ofless than 7 and preferably from 3 to 7. The pH is preferably lowered bythe addition of an acid, preferably acidic acid to the slurry. Inparticularly preferred embodiments the second slurry is comminuted toresult in substantially all of the solids having particle sizes of lessthan 14 micrometers in average diameter.

The second supported rhodium group component and second platinumcomponent in the resulting second mixture are converted to a waterinsoluble form. The platinum and rhodium components can be converted toinsoluble form thermally, chemically or by calcining. The second layeris preferably thermally fixed, preferably at about 50° C. to 550° C. forfrom 0.5 to 2.0 hours.

The first slurry containing a supported palladium component and thesecond slurry containing a supported rhodium component can be mixed toform a complete slurry. Additives such as oxygen storage components,stabilizers, rare earth metal components, and zirconium components andthe like can be added either to the first slurry, to the second slurryor the complete slurry. Preferably the additional additives are added tothe first or second slurry prior to a step of co-minuting the slurry.

Each of the first and second slurries useful for the presentcompositions can also be prepared by the method in disclosed in U.S.Pat. No. 4,134,860 (incorporated by reference) generally recited asfollows.

A finely-divided, high surface area, refractory oxide support iscontacted with a solution of a water-soluble, catalytically-promotingmetal component, preferably containing one or more platinum group metalcomponents, to provide a mixture which is essentially devoid of free orunabsorbed liquid. The catalytically-promoting platinum group metalcomponent of the solid, finely-divided mixture can be converted at thispoint in the process into an essentially water-insoluble form while themixture remains essentially free of unabsorbed liquid. This process canbe accomplished by employing a refractory oxide support, e.g., alumina,including stabilized aluminas, which is sufficiently dry to absorbessentially all of the solution containing the catalytically-promotingmetal component, i.e., the amounts of the solution and the support, aswell as the moisture content of the latter, are such that their mixturehas an essential absence of free or unabsorbed solution when theaddition of the catalytically-promoting metal component is complete. Thecomposite remains essentially dry, i.e. it has substantially no separateor free liquid phase. During the latter process the metal component canbe fixed on the support.

After the catalytically-promoting metal solution and high arearefractory oxide support are combined the catalytically-promoting metalcomponent can be fixed on the support, i.e., converted to essentiallywater-insoluble form, while the composite remains essentially devoid offree or unabsorbed aqueous medium. The conversion may be effectedchemically, by treatment with a gas such as hydrogen sulfide or hydrogenor with a liquid such as acetic acid or other agents which may be inliquid form, especially an aqueous solution, e.g. hydrazine. The amountof liquid used, however, is not sufficient for the composite to containany significant or substantial amount of free or unabsorbed liquidduring the fixing of the catalytically-promoting metal on the support.The fixing treatment may be with a reactive gas or one which isessentially inert; for example, the fixing may be accomplished bycalcining the composite in air or other gas which may be reactive withthe catalytically-promoting metal component or essentially inert. Theresulting insoluble or fixed catalytically-promoting metal component maybe present as a sulfide, oxide, elemental metal or in other forms. Whena plurality of catalytically-promoting metal components are deposited ona support, fixing may be employed after each metal component depositionor after deposition of a plurality of such metal components.

The first and second slurries containing the fixed,catalytically-promoting metal component can be comminuted as a slurrywhich is preferably acidic, to provide solid particles that areadvantageously primarily of a size of about 5 to 15 microns. Theslurries are mixed to result in a complete slurry which can be used tocoat a macrosize carrier, typically having a low surface area, and thecomposite is dried and may be calcined. In these catalysts the compositeof the catalytically-promoting metal component and high area supportexhibits strong adherence to the carrier, even when the latter isessentially non-porous as may be the case with, for example, metalliccarriers, and the catalysts have very good catalytic activity and lifewhen employed under strenuous reaction conditions. Each of the first andsecond slurries can be mixed to form a complete slurry and applied as alayer supported on a substrate carrier and calcined of the presentinvention.

The method provides compositions of uniform and certaincatalytically-promoting metal content since essentially all of theplatinum group metal component thereby added to the preparation systemremains in the catalyst, and the compositions contain essentially thecalculated amount of the active catalytically-promoting metalcomponents. In some instances a plurality of catalytically-active metalcomponents may be deposited simultaneously or sequentially on a givenrefractory oxide support. The intimate mixing of separately preparedcatalytically-promoting metal component refractory oxide composites ofdifferent composition made by the procedure of this invention, enablesthe manufacture of a variety of catalyst whose metal content may beclosely controlled and selected for particular catalytic effects. Thecomposition may have a platinum group metal component on a portion ofthe refractory oxide particles, and a base metal component on adifferent portion of the refractory oxide particles. It is, therefore,apparent that this process is highly advantageous in that it providescatalysts which can be readily varied and closely controlled incomposition.

The comminution of the first and second slurries can be accomplished ina ball mill or other suitable equipment, and the solids content of theslurry my be, for instance, about 20 to 60 weight percent, preferablyabout 35 to 45 weight percent. The pH of each slurry is preferably belowabout 6 and acidity may be supplied by the use of a minor amount of awater-soluble organic or inorganic acid or other water-soluble acidiccompounds. Thus the acid employed may be hydrochloric or nitric acid, ormore preferably a lower fatty acid such as acetic acid, which may besubstituted with, for example, chlorine as in the case oftrichloroacetic acid. The use of fatty acids may serve to minimize anyloss of platinum group metal from the support.

In making catalysts by this invention, the catalytically-activecomposition derived from the first and second slurries, having fixed orwater-insoluble catalytically-promoting metal components and high areasupports can be combined with a macrosize carrier, preferably of lowtotal surface area. In order to deposit the catalytically-promotinggroup metal-support composite on the macrosized carrier, one or morecomminuted complete slurries are applied to the carrier in any desiredmanner. Thus the carrier may be dipped or sprayed with the completeslurry, until the appropriate amount of slurry is on the carrier. Theslurry employed in depositing the catalytically-promoting metalcomponent-high area support composite on the carrier will often containabout 20 to 60 weight percent of finely-divided solids, preferably about35 to 45 weight percent. Alternatively, the catalyst composition can beused in the form of a self-supporting structure such as a pellet. Thecomposition can be prepared and formed into pellets by known means.

The comminuted catalytically-promoting metal component-high surface areasupport composite can be deposited on the carrier such as a metal orceramic honeycomb in a desired amount, for example, the composite maycomprise about 2 to 30 weight percent of the coated carrier, and ispreferably about 5 to 20 weight percent. The composite deposited on thecarrier is generally formed as a coated layer over most, if not all, ofthe surfaces of the carrier contacted. The combined structure may bedried and calcined, preferably at a temperature of at least about 250°C., but not so high as to unduly destroy the high area of the refractoryoxide support, unless such is desired in a given situation.

The carriers useful for the catalysts made by this invention may bemetallic in nature and be composed of one or more metals or metalalloys. The metallic carriers may be in various shapes such as pelletsor in monolithic form. Preferred metallic supports include theheat-resistant, base-metal alloys, especially those in which iron is asubstantial or major component. Such alloys may contain one or more ofnickel, chromium, and aluminum, and the total of these metals mayadvantageously comprise at least about 15 weight percent of the alloy,for instance, about 10 to 25 weight percent of chromium, about 1 to 8weight percent of aluminum and 0 to about 20 weight percent of nickel.The preferred alloys may contain small or trace amounts of one or moreother metals such as molybdenum, copper, silicon, niobium, titanium andthe like. The surfaces of the metal carriers may be oxidized at quiteelevated temperatures, e.g. at least about 1000° C., to improve thecorrosion resistance of the alloy by forming an oxide layer on thesurface of carrier which is greater in thickness and of higher surfacearea than that resulting from ambient temperature oxidation. Theprovision of the oxidized or extended surface on the alloy carrier byhigh temperature oxidation may enhance the adherence of the refractoryoxide support and catalytically-promoting metal components to thecarrier.

Any suitable carrier may be employed, such as a monolithic carrier ofthe type having a plurality of fine, parallel gas flow passagesextending therethrough from an inlet or an outlet face of the carrier,so that the passages are open to fluid flow therethrough. The passages,which are essentially straight from their fluid inlet to their fluidoutlet, are defined by walls on which the catalytic material is coatedas a "washcoat" so that the gases flowing through the passages contactthe catalytic material. The flow passages of the monolithic carrier arethin-walled channels which can be of any suitable cross-sectional shapeand size such as trapezoidal, rectangular, square, sinusoidal,hexagonal, oval, circular. Such structures may contain from about 60 toabout 600 or more gas inlet openings ("cells") per square inch of crosssection. The ceramic carrier may be made of any suitable refractorymaterial, for example, cordierite, cordierite-alpha alumina, siliconnitride, zircon mullite, spodumene, alumina-silica magnesia, zirconsilicate, sillimanite, magnesium silicates, zircon, petalite, alphaalumina and aluminosilicates. The metallic honeycomb may be made of arefractory metal such as a stainless steel or other suitable iron basedcorrosion resistant alloys.

Such monolithic carriers may contain up to about 600 or more flowchannels ("cells") per square inch of cross section, although far fewermay be used. For example, the carrier may have from about 60 to 600,more usually from about 200 to 400, cells per square inch ("cpsi").

The catalytic compositions made by the present invention can be employedto promote chemical reactions, such as reductions, methanations andespecially the oxidation of carbonaceous materials, e.g., carbonmonoxide, hydrocarbons, oxygen-containing organic compounds, and thelike, to products having a higher weight percentage of oxygen permolecule such as intermediate oxidation products, carbon dioxide andwater, the latter two materials being relatively innocuous materialsfrom an air pollution standpoint. Advantageously, the catalyticcompositions can be used to provide removal from gaseous exhausteffluents of uncombusted or partially combusted carbonaceous fuelcomponents such as carbon monoxide, hydrocarbons, and intermediateoxidation products composed primarily of carbon, hydrogen and oxygen, ornitrogen oxides. Although some oxidation or reduction reactions mayoccur at relatively low temperatures, they are often conducted atelevated temperatures of, for instance, at least about 150° C.,preferably about 200° to 900° C., and generally with the feedstock inthe vapor phase. The materials which are subject to oxidation generallycontain carbon, and may, therefore, be termed carbonaceous, whether theyare organic or inorganic in nature. The catalysts are thus useful inpromoting the oxidation of hydrocarbons, oxygen-containing organiccomponents, and carbon monoxide, and the reduction of nitrogen oxides.These types of materials may be present in exhaust gases from thecombustion of carbonaceous fuels, and the catalysts are useful inpromoting the oxidation or reduction of materials in such effluents. Theexhaust from internal combustion engines operating on hydrocarbon fuels,as well as other waste gases, can be oxidized by contact with thecatalyst and molecular oxygen which may be present in the gas stream aspart of the effluent, or may be added as air or other desired formhaving a greater or lesser oxygen concentration. The products from theoxidation contain a greater weight ratio of oxygen to carbon than in thefeed material subjected to oxidation. Many such reaction systems areknown in the art.

The present invention is illustrated further by the following exampleswhich are not intended to limit the scope of this invention.

EXAMPLES 1 AND 2

Sample Preparation

Examples 1-4 described below are two single coat Pd/Rh catalyst bricks.The difference between the Example 1 and Example 2 is that Example 2contains an additional 0.2 g/ft³ of CeO₂ (impregnated as Ce-Nitrate).Examples 3 and 4 are similar to Examples 1 and 2 with differencesreviewed below.

Examples 1 to 4 have a 50 g/ft³ precious metal loading with a Pd/Rhratio of 9/1 in a single layer.

Example 1

In a Planetary Mixer, 66 g of an aqueous solution of 10 wt. % Rh(NO₃)₃was mixed with 230 g of a lower surface area alumina, having a surfacearea specified to be about 160 m² /g and an average particle size ofabout 30 micrometers, and 690 g of a co-formed ceria-zirconia having asurface area specified to be about 60 m² /g and an average particle sizeof about 5 micrometers. The relative amounts of the alumina and theco-formed ceria-zirconia was 1:3 by weight. This rhodium containingpowder mixture was air dried in an oven at 110° C. for one hour to fixthe rhodium on the supports.

In a separate Planetary Mixer, 288 g of an aqueous solution of 21 wt. %Pd(NO₃)₃ was mixed with 920 g of lower surface area alumina, and wasmixed with 1150 g of higher surface area alumina, having a surface areaspecified to be about 250 m² /g and an average particle size of about 40micrometers. The relative amounts of the lower surface area alumina (160m² /g) and higher surface area alumina (250 m² /g) was 4:5 by weight.This palladium containing powder mixture was air dried in an ovenovernight at 110° C. to fix the palladium on the support. The mixturewas then milled for 12 hours using a Roalox mill jar. Aqueous solutionsof Sr-Nitrate (471 g of a 50% solution), La-Nitrate (847 g of a 37%solution) and Zr-Acetate (1150 g of a 20% solution) were added prior toballmilling.

After the Pd mixture particle size was 90% less than 10 micron asmeasured using a Brinkman Particle Size Analyzer, the rhodium containingmixture was added to form a complete slurry and the ball millingcontinued for another hour. The resulting particle size distribution ofthe rhodium containing particles is estimated to be about 90% of theparticles below about 25 micrometers. The pH of the slurry was 3.2 andthe viscosity was over 1000 cpi as measured according to BrookfieldViscometer. Acetic acid was added to lower the viscosity to 970 cpi. Theresulting slurry had a solid concentration of 33% and a pH at 2.9. Acordierite monolith support containing about 400 flow passages persquare inch of cross section was dipped into the complete washcoatslurry. The monolith had an circular cross section with a diameter of 4inches and a length of 6 inches. The excess liquid was blown off of themonolith with compressed air. The resultant catalyzed monolith was driedat 100° C. for about 20 minutes and calcined in an oven for one hour at550° C. The resulting monolith contained 45 g/ft³ palladium, 5 g/ft³rhodium, 0.5 g/in³ lower surface area alumina, 0.5 g/in³ higher surfacearea alumina, 0.30 g/in³ co-formed ceria-zirconia composite, 0.1 g/in³ZrO₂, 0.1 g/in³ La₂ O₃, 0.1 g/in³ SrO.

Example 2

Example 2 was prepared similarly to Example 1 except that Ce nitrate wasadded into half the Pd-frit prior to the ball milling. The dry gain forthe CeO₂ was 0.2 g/in³. The precious metal distribution was changed to66 g of the rhodium nitrate solution on 690 g of the co-formedceria-zirconia, and 230 g of the lower surface area alumina; 288 g ofthe palladium nitrate solution onto 920 g of the lower surface areaalumina at 1150 g of the higher surface area alumina.

The Test Results

The Example 1 and 2 catalysts shown above were aged under RAT-A cyclesfor 30 hours using unleaded premium fuel. RAT-A is a rapid aging cyclewith an inlet exhaust gas temperature of 800° C. and air/fuel ratio at astoichiometric value for 40 seconds. The CO concentration is thenincreased to 3% for 6 seconds. O₂ concentration is then increased to 3%for 10 seconds while 3% CO is maintained so that the overall exhaustcondition is lean (air rich). After that, CO concentration was adjustedso that the overall exhaust gas condition is lean for 4 seconds with 3%O₂ maintained. The above cycles are repeated during the entire agingperiod. After aging, these catalysts were tested for FTP using a MY96Saturn vehicle. Results are shown below:

    ______________________________________                                        No             THC g/mile  CO, g/mile                                                                            NOx, g/mile                                ______________________________________                                        1.             0.142       0.900   0.280                                      2.                                           0.160                                     Engine Out                                                                                    1.780                                                                                             1.990                            ______________________________________                                    

EXAMPLES 3 AND 4

Example 1 was reproduced as Examples 3 with some minor changes. First,no oven drying was used after each impregnation. Second, a new batch ofco-formed ceria-zirconia was used. This new co-formed ceria-zirconia hasa surface area similar to the co-formed ceria-zirconia of Example 1.Instead of precipitating separately, the ceria-zirconia wascoprecipitated. Third, zirconium hydroxide paste was used, instead ofzirconium acetate, to reduce leaching of the Pd from the alumina.Fourth, the Rh slurry was milled separately, then mixed with milled Pdslurry during the coating. The pH value was 3.9 and the viscosity was850 cpi with a solids concentration of 40.5%. The same type cordieritesubstrates were used. Example 4 was made identical to Example 3 exceptthat the ceria nitrate was added in half the palladium frit prior toballmilling. The other half of the palladium frit has no ceria added.The Example 3 and 4 catalysts were aged under RAT-A cycles for 30 hoursusing unleaded premium fuel. After aging, this time, instead of using aSaturn they were evaluated using a MY95 Honda Accord. FTP conditionswere: stoichiometric A/F ratio, VHSV: 52K, Temperature: 500° C., at 0.3A/F and 1 Hz. Federal Testing Procedure (FTP) results are listed below:

    ______________________________________                                        No    THC, g/mile   CO, g/mile                                                                              NOx, g/mile                                     ______________________________________                                        3.    0.100         2.030     0.230                                           4.          0.110                        0.210                                ______________________________________                                    

The above results show that the first slurry can be added as twoslurries, one with Pd plus Ce (Nitrate) and the other with Pd without CeNitrate. Although THC is not as low for Example 4, CO and NOx amountswere improved. This illustrates the segregation of the slurries of thefirst precious metal and takes advantage of minimizing the interactionof palladium and ceria. Ceria was present for CO and NOx performancewhile it was not used with part of Pd slurry to achieve satisfactory HClight off.

Examples 2 and 3 were evaluated using Sweep test to establish a linkbetween the FTP tests using different vehicles. A Sweep test is a testto measure the catalyst performance under simulated exhaust gasconditions in which the air/fuel ratio is at a constant swing betweenrich and lean. However, both the amplitude (±air/fuel ratio) andfrequency (Hz) can be controlled, as can the inlet temperature. Thecurrent example is evaluated under the following conditions. At thestoichiometric A/F ratio, VHSV: 52K, Temperature: 500° C., at 0.3 A/Fand 1 Hz.

Results shown below indicate that Example 3 made via the secondpreparation method performs better than Example 2 made using the firstpreparation technique. Accordingly, it is desirable to minimize liquidin the slurry when adding additives, particularly additives when theprecious metal component has not been fixed. Secondly, it is preferredto mill the first and second slurries separately.

    ______________________________________                                              THC,           CO,     NOx,                                             No     % conv.              % conv.                                                                                % conv.                                  ______________________________________                                         2.   84.000         95.000  92.000                                           3.        89.000                       96.000                                 ______________________________________                                    

What is claimed:
 1. A catalyst composition comprising:at least one firstsupport; at least one first precious metal component supported on atleast one first support; at least one second support; at least onesecond precious metal component supported on at least one secondsupport; wherethe total amount of the first precious metal componentcomprises from 1 to 99 weight percent based on the total of the firstand second precious metal components, and the average particle size ofthe second support is greater than the average particle size of thefirst support, wherein the first support has an average particle size of50% of the particles below 10 micrometers, and the second support has anaverage particle size of 50% of the particles below 30 micrometers.
 2. Amethod comprising the steps of:contacting a gas comprising nitrogenoxide, carbon monoxide and/or hydrocarbon with a catalyst compositioncomprising: at least one first support; at least one first preciousmetal component supported on at least one first support; at least onesecond support; at least one second precious metal component supportedon at least one second support; where the total amount of the firstprecious metal component comprises from 1 to 99 weight percent based onthe total of the first and second precious metal components, and theaverage particle size of the second support is greater than the averageparticle size of the first support, wherein the first support has anaverage particle size of 50% of the particles below 10 micrometers, andthe second support has an average particle size of 50% of the particlesbelow 30 micrometers.
 3. The catalyst composition of claims 1 or 2wherein the average particle size of the second support is at leastabout one micrometer greater than the average particle size of the firstsupport.
 4. The catalyst composition of claim 3 wherein the averageparticle size of the second support is at least about two micrometersgreater than the average particle size of the first support.
 5. Thecatalyst composition of claims 1 or 2 wherein the at least one of thefirst precious metal components and the at least one second preciousmetal components, comprises at least one precious metal component notpresent in the other precious metal component.
 6. The catalystcomposition of claims 1 or 2 wherein the at least one of the firstprecious metal components comprises a palladium component and the atleast one second precious metal components comprises a rhodiumcomponent.
 7. The catalyst composition as recited in claims 1 or 2wherein the first and second supports are the same or different and arecompounds selected from the group consisting of silica, alumina andtitania compounds.
 8. The catalyst composition as recited in claims 1 or2 wherein the first and second supports are the same or different andare activated compounds selected from the group consisting of alumina,silica, silica-alumina, alumino-silicates, alumina-zirconia,alumina-chromia, and alumina-ceria.
 9. The catalyst composition asrecited in claim 8 wherein the first and second supports are activatedalumina.
 10. The catalyst composition as recited in claims 1 or 2further comprising a nickel or iron component.
 11. The catalystcomposition of claims 1 or 2 further comprises at least one componentselected from the group consisting of:at least one rare earth metalcomponent; an oxygen storage composition; at least one first stabilizer;and a compound containing zirconium.
 12. The catalyst composition asrecited in claim 11 wherein at least one of said rare earth metalcomponent is selected from the group consisting of lanthanum componentsand neodymium components.
 13. The catalyst composition as recited inclaim 12 wherein the at least one rare earth component is derived fromneodymium.
 14. The catalyst composition as recited in claim 12 whereinthe at least one rare earth component is derived from lanthanum.
 15. Thecatalyst composition as recited in claim 11 wherein the oxygen storagecomposition is in bulk form.
 16. The catalyst composition as recited inclaim 15 wherein the oxygen storage component is selected from the groupconsisting of cerium and praseodymium compounds.
 17. The catalystcomposition as recited in claim 16 wherein the oxygen storage componentis ceria.
 18. The catalyst composition as recited in claim 11 where theoxygen storage composition comprises a refractory oxide and a oxygenstorage component.
 19. The catalyst composition as recited in claim 18wherein the oxygen storage composition comprises a ceria oxygen storagecomponent and zirconia refractory oxide composite.
 20. The catalystcomposition as recited in claim 11 wherein the stabilizer is at leastone alkaline earth metal component derived from a metal selected fromthe group consisting of magnesium, barium, calcium and strontium. 21.The catalyst composition as recited in claim 20 wherein the at least onealkaline earth metal component is derived from a metal selected from thegroup consisting of strontium and barium.
 22. The catalyst compositionas recited in claim 21 wherein the first alkaline earth metal componentis barium oxide.
 23. The catalyst composition as recited in claim 21wherein the alkaline earth metal component is strontium oxide.
 24. Thecatalyst composition as recited in claim 11 further comprising aparticulate composite of zirconia compound and rare earth oxide.
 25. Thecatalyst composition as recited in claim 24 wherein the rare earth oxideis ceria and, optionally, further comprises lanthana, neodymia andmixtures thereof.
 26. The catalyst composition as recited in claims 1 or2 wherein there is:from about 0.001 to about 0.3 g/in³ of at least onefirst precious metal component; from about 0.15 to about 2.0 g/in³ ofthe first support; from about 0.001 to about 0.3 g/in³ of at least onesecond precious metal component; from about 0.15 g/in³ to about 1.5g/in³ of the second support; from about 0.025 to about 0.5 g/in³ of atleast one alkaline earth metal components; from about 0.025 to about 0.5g/in³ of the zirconium containing component; and from about 0.025 toabout 0.5 g/in³ of at least one rare earth metal component selected fromthe group consisting of ceria metal components, lanthanum metalcomponents and neodymium metal components.
 27. The catalyst compositionas recited in claim 26 wherein the composition comprises from about0.025 g/in³ to about 0.5 g/in³ of a nickel component.
 28. The catalystcomposition as recited in claim 27 further comprising from about 0.1g/in³ to about 1.0 g/in³ of a particulate composite of zirconia andceria and optionally further comprising a rare earth component selectedfrom the group consisting of lanthana, neodymia and mixtures thereof.29. The catalyst composition as recited in claim 28 wherein theparticulate composite of zirconia and ceria comprises 60 to 90 wt. %zirconia, 10 to 30 wt. % ceria and from 0 to 10 wt % rare earth oxidescomprising lanthana, neodymia, yttria and mixtures thereof.
 30. Thecatalyst composition as recited in claims 1 or 2 in the form of apellet.
 31. The catalyst composition as recited in claims 1 or 2 whereinthe first and/or second support comprises a mixture of a rare earthcontaining support having a refractory oxide and a rare earth metalcomponent impregnated onto the refractory oxide and a non-rare earthcontaining support having a refractory oxide without a rare earth metalcomponent impregnated thereon.
 32. The catalyst composition as recitedin claim 31 wherein the first and/or second support comprises a mixtureof a refractory oxide support which further comprises ceria, and arefractory oxide support not containing ceria.
 33. The catalystcomposition as recited in claim 32 wherein the first support comprises amixture of a refractory oxide plus ceria and a refractory without ceria.34. The catalyst composition of claims 1 or 2 wherein the first supporthas an average particle size of 90% of the particles below 10micrometers, and the second support has an average particle size of 80%of the particles below 30 micrometers.
 35. The catalyst composition ofclaim 34 wherein the first support has an average particle size of 90%of the particles below 8 micrometers, and the second support has anaverage particle size of 80% of the particles below 15 micrometers.